German Chancellor Angela Merkel has said an EU plan to give visa-free travel to Turkey by July 1 will have to be delayed amid concerns that Ankara will not meet key conditions of the deal in time.

The bloc has demanded that Turkey implement a series of reforms, including changing its terrorism laws, to meet EU political requirements. Ankara has so far refused to carry out the reforms.

Merkel said the requirements for the deal had "not been able to be completed yet" in a press conference after a meeting with Turkish President Recep Tayyip Erdogan on the sidelines of a UN humanitarian summit in Istanbul.

"We must do everything that we can to continue to discuss, as it is likely that by July 1 certain things will not be put in place," she said.

Merkel told Erdogan of her "deep concern" over a law lifting the immunity of Turkish lawmakers that critics believe is aimed at evicting pro-Kurdish lawmakers from parliament.

The visa deal is part of an agreement between Turkey and the EU to deal with the migrant crisis in Europe.